The root cause of hydrogen induced changes in optical transmission of vanadium

The changes in the optical transmission of thin vanadium layers upon hydrogen absorption are found to be dominated by the volume changes of the layers and not directly linked to concentration. This effect is demonstrated by utilising the difference in the hydrogen induced expansion of V layers in Fe/V and Cr/V superlattices. Hydrogen resides solely in the vanadium layers in these superlattices, while occupying different sites, causing different lattice expansion. Quantitative agreement is obtained between the experimental results and first principle density functional calculations.